A new study shows epidurals can actually lengthen labor longer than previously thought, bringing to question current standards 'normal labor.'

Using epidurals for pain relief during a baby's delivery may prolong labor more than previously thought, a new study finds.

In the study, the researchers looked at more than 42,000 women in California who delivered vaginally between 1976 and 2008, and compared the length of the second stage of labor, which is the time it takes for "pushing" the baby out after the cervix has fully opened, among women who had received epidurals and those who hadn't.

Although it was thought that epidurals lengthen labor by about one hour, the researchers found that women who had epidurals actually took two to three hours longer to get through the second stage of labor, compared with women who hadn't received this pain medication, according to the study, published Wednesday (Feb. 5) in the journal Obstetrics & Gynecology.

The findings could affect doctors' decisions to perform cesarean-section deliveries, the researchers said. Some C-sections are performed because labor is judged as taking too long. The new findings suggest that for women who receive an epidural, doctors may be able to wait a little longer before opting for the surgery.

"When epidural is used, it may be normal for labor to take two hours longer, and physicians don't necessarily have to intervene, as long as women are progressing and the baby is OK," said Dr. Yvonne Cheng, one of the researchers on the study and an obstetrician at University of California, San Francisco. [8 Odd Changes That Happen During Pregnancy]

Current definitions of "normal" labor account for one additional hour for women who have epidurals. This means that women who take longer than that may get a label of "prolonged second stage," and their doctors may choose to intervene by performing a C-section, or use either a vacuum device or forceps to help the baby out of the birth canal.

"Although the American College of Obstetricians and Gynecologists specifically says that the doctor doesn't have to intervene based on the passage of time alone, it is still kind of a gray zone," Cheng said.

Most definitions of what is normal during labor are based on norms established by Dr. Emmanuel Friedman in the 1950s, and may not properly fit the contemporary population, experts say. Today, women and babies are heavier on average, more women give birth at older ages and more women use epidurals.

"In the Friedman population, epidural was used in 8 percent of the population," Cheng said. "Today, it's closer to 60 percent."

In the new study, in women who were having a baby for the first time, the second stage of labor took 336 minutes with epidural, and 197 minutes without epidural — a difference of 2 hours and 19 minutes.

For women who had given birth before, the length of second stage was 255 minutes with epidural, and 81 minutes without epidural — a difference of 2 hours and 54 minutes, the study found.

The researchers cautioned that labor norms should not be established based on their study alone, and that more research is required to re-establish what should be considered normal labor in the contemporary population.

It is not fully understood exactly why epidurals prolong labor, but experts speculate that the drug relaxes the pelvic muscles and the woman has less urge to push.